Gasoline, used as an automotive fuel in many automotive vehicles, is a volatile liquid subject to potentially rapid evaporation, in response to diurnal variations in the ambient temperature. Thus, the fuel contained in automobile gas tanks presents a major source of potential evaporative emission of hydrocarbons into the atmosphere. Such emissions from vehicles are termed ‘evaporative emissions’. The engine produces such vapors even while being is turned off.
Industry's response to this potential problem has been the incorporation of evaporative emission control systems (EVAP) into automobiles, to prevent fuel vapor from being discharged into the atmosphere. EVAP systems include a canister (the carbon canister) containing adsorbent carbon) that traps fuel vapor. Periodically, a purge cycle feeds the captured vapor to the intake manifold for combustion, thus reducing evaporative emissions.
Hybrid electric vehicles, including plug-in hybrid electric vehicles (HEV's or PHEV's), pose a particular problem for effectively controlling evaporative emissions with this kind of system. Although hybrid vehicles have been proposed and introduced having a number of forms, these designs share the characteristic of providing a combustion engine as backup to an electric motor. Primary power is provided by the electric motor, and careful attention to charging cycles can result in an operating profile in which the engine is only run for short periods. Systems in which the engine is only operated once or twice every few weeks are not uncommon. Purging the carbon canister can only occur when the engine is running, of course, and if the canister is not purged, the carbon pellets can become saturated, after which hydrocarbons will escape to the atmosphere, causing pollution.
Over time, the canister pellets become loaded with hydrocarbons. Adsorption occurs during refueling operations, diurnal temperature variations, and running vapor losses. The primary loading source is refueling, as the fuel tank is sealed to contain diurnal and running vapor generation. If not purged for some time, the canister can reach saturation, which presents a risk that additional vapor can result in vapor escaping to the atmosphere. Therefore, identifying the loading state of the canister is a key step to ensure timely purging.
Conventional automotive vehicles use an oxygen sensor (O2 sensor) to determine the canister's loading state. Being located in the exhaust stream, these sensors identify changes in the air-fuel ratio during purging, which allows the control system to infer the state of canister loading. PHEVs, however, generally experience limited engine running time, which in turn limits the utility of that method. Hydrocarbon sensors provide a substitute method, but they are comparatively expensive.
Considering the problems mentioned above, and other shortcomings in the art, there exists a need for an efficient method and system for identifying the state of loading of a carbon canister within an EVAP system of a PHEV.